Methods for making antireflection coatings for heat-treatable inorganic substrates

ABSTRACT

The invention includes a bilayer antireflection coating, multilayer antireflection coatings, and a method for making a multilayer antireflective coated substrate. The bilayer and multilayer coatings include layers having metallic oxides, at least one layer in each of such coatings having as a material with an index of refraction at least about 1.90, a layer comprising oxides of zirconium such that when subjected to heat treatments, such as tempering, the coatings substantially retain their optical properties. The coatings further include layers having materials of low index of refraction which include oxides of aluminum which provide coating stability and increased precursor storage life. In the method, the layers are applied by successive dip coating techniques using solvent-based solutions and the resulting coatings can be used to coat large sized substrates which can be later cut, trimmed and/or otherwise shaped and later heat treated with substantial retention of optical properties.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 09/201,599filed on Nov. 30, 1998, entitled “Antireflection Coatings And OtherMultilayer Optical Coatings For Heat-Treatable Inorganic Substrates AndMethods For Making Same” now U.S. Pat. No. 6,410,173. The entiredisclosure of application Ser. No. 09/201,599 as filed is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

Antireflection coatings are used for a variety of substrates,particularly glass. Broad band antireflection coatings are typicallydesigned to minimize reflection throughout the visible range of thespectrum. There are many ways to apply such coatings to glass, includingdip coating using sol gel techniques and more costly vacuum techniquessuch as sputtering.

Safety glass for automobiles, picture frames, display cases and the likeis typically formed by subjecting cut or shaped glass to a hightemperature heat treatment, followed by rapid cooling. The processinduces stress in the glass which thereby contributes to enhancing themechanical stability of the glass forming safety glass. Such glass, dueto its enhanced properties, cannot be cut, trimmed or otherwisemechanically treated after tempering. As such, glass substrates are cut,shaped and/or trimmed prior to tempering.

It is advantageous in many safety glass manufacture to provide coatingsto minimize glare and the like. However, many difficulties areencountered in applying antireflection or other optical coatings toglass which is to be tempered. In the prior art, such coatings cannot bepractically applied prior to tempering because the tempering processcreates disadvantages with respect to resulting optical properties.During tempering, thinning of the coating layers occurs, typically withrespect to the outer layer of a multilayer antireflection coating. Theouter layer may be burned off and/or the entire coating systemdistorted. Further, the index of refraction may be affected due tochanges in the crystal structure or density changes of some materialsduring tempering. Such changes affect the optical properties of thewhole system. Titanium dioxides, which are commonly used as a middlelayer in three layer antireflection coatings, are significantly affectedby tempering.

In addition to the above-noted problems, the quick cooling used in thetempering process which induces the desired stress within the glass,unfortunately also induces undesirable stress into the antireflection orother optical coating subjected to tempering. The stress in the coating,however, is not beneficial and often leads to disintegration, cracks ormicrocracks. The coating will appear hazy as a result, or may becompletely destroyed such that it cracks or flakes off.

Because of the disadvantageous results of tempering coated glass,antireflection coatings have been applied using various coatingtechniques after the glass has been tempered. Unfortunately, this meansthat large pieces of commercial glass must first be cut and shaped, thentempered. As a result, coating is done on smaller, pre-cut pieces oftempered glass. This process is time-consuming, inefficient, and,therefore, tends to be uneconomical.

In attempting to coat already cut tempered glass with antireflectioncoatings, sophisticated vacuum coating techniques are typicallyrequired, because practical dip-coating techniques used for standardantireflection coated glass have not been used successfully in the artof making tempered glass. Application of an antireflection coating usingdip coating techniques involves heat treatments to form the coatingswhich cause relief in the stress created by tempering the glass. Reliefof the stress resulting from tempering contributes to deteriorating themechanical properties provided by tempering. While there has been anattempt to form tempered glass using dip coating which involvesestimating the changes in optical properties and trying to compensatefor them prior to heat treatment, such methods lack sufficient qualitycontrol and do not maintain adequate reflection color. As such, dipcoating, while cost effective and practical for use in coating standardglass, has not been practically and effectively used for coatingtempered glass or other heat treated inorganic substrates to providecoatings of adequate reflection color, and of reproducible opticalquality.

As a result of the inability to use dip-coating techniques, it has beennecessary to use techniques such as cathodic sputtering to applyantireflection coatings for tempered glass. Examples of such techniquesare described in U.S. Pat. Nos. 5,059,295 and 5,028,759 of Finley.

While sputtering allows for application of antireflection coatings formaking tempered glass, there is still a need in the art for aneconomical process for large scale formation of antireflection coatedglass which can be cut and used to form tempered glass. There is furthera need in the art for an economical method for coating glass prior tocutting and trimming for tempering such that larger pieces of glass maybe coated while still providing tempered glass of optical quality.

BRIEF SUMMARY OF THE INVENTION

The invention includes a multilayer antireflection coating for use incoating a heat treatable inorganic substrate comprising an inner layerfor contact with an inorganic substrate having at least two differentmetallic oxides and being capable of providing an index of refraction offrom about 1.54 to about 1.90 after curing. The coating further includesa middle layer on the inner layer comprising an oxide of zirconium andbeing capable of providing an index of refraction of at least about 1.90after curing and an outer layer on the middle layer comprising at leastone metallic oxide and being capable of providing an index of refractionof about 1.54 or less after curing. The optical properties of themultilayer antireflection coating are substantially retained after themultilayer antireflection coating is applied to an inorganic substrateand subsequently subjected to a heat treatment.

The invention further includes a bilayer antireflection coating for usein coating a heat treatable inorganic substrate comprising an innerlayer for contact with an inorganic substrate comprising an oxide ofzirconium and at least one oxide of a metal different from zirconium,wherein the inner layer is capable of providing an index of refractionof from about 1.54 to about 1.90 after curing, and an outer layer on theinner layer having at least one metallic oxide and being capable ofproviding an index of refraction of about 1.54 or less after curing. Theoptical properties of the bilayer antireflection coating aresubstantially retained after the bilayer antireflection coating isapplied to an inorganic substrate and subsequently subjected to a heattreatment.

Also within the invention is a multilayer antireflection coating for usein coating a heat treatable inorganic substrate comprising an innerlayer comprising an oxide of zirconium and being capable of providing anindex of refraction of at least about 1.90 after curing; a first middlelayer on the inner layer having at least one metallic oxide and beingcapable of providing an index of refraction of about 1.54 or less aftercuring; a second middle layer on the first middle layer comprising anoxide of zirconium and being capable of providing an index of refractionof at least about 1.90 after curing; and (d) an outer layer on thesecond middle layer having at least one metallic oxide and being capableof providing an index of refraction of about 1.54 or less after curing.The optical properties of the multilayer antireflection coating aresubstantially retained after the multilayer antireflection coating isapplied to an inorganic substrate and subsequently subjected to a heattreatment.

A method for making a heat treated antireflective coated inorganicsubstrate is also provided. The method comprises coating an inorganicsubstrate with an inner layer comprising a mixture of at least one firstoxide of a metal selected from the group consisting of titanium,zirconium, lanthanum, tantalum, and niobium and at least one secondoxide of a metal selected from the group consisting of silicon, andaluminum to form an inner layer. The inner layer is coated with a middlelayer comprising an oxide of zirconium. The middle layer is coated withan outer layer of at least one oxide of a metal selected from the groupconsisting of silicon, and aluminum; and the coated inorganic substrateis heat treated.

DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunction with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentswhich are presently preferred. It should be understood, however, thatthe invention is not limited to the precise arrangements andinstrumentalities shown. In the drawings, like numerals are used toindicate like elements throughout. In the drawings:

FIG. 1 is a perspective view of a shaped antireflection coated piece ofsafety glass formed according to one embodiment of the invention;

FIG. 2 is an enlarged, partially broken cross-sectional view of aportion of the three-layer multilayer antireflection coating of FIG. 1;

FIG. 3 is an enlarged, partially broken cross-sectional view of aportion of a bilayer antireflection coating formed in accordance with anembodiment of the invention;

FIG. 4 is an enlarged, partially broken cross-sectional view of aportion of a four-layer multilayer antireflection coating formed inaccordance with one embodiment of the invention; and

FIG. 5 is a graphical representation of the effect of tempering on amultilayer antireflection coating formed in accordance with Example 4.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described with respect to itspreferred embodiments. The following description should not beconsidered limiting. Words such as “left” and “right,” “upper” and“lower” and “inner” and “outer” designate directions in the drawings towhich reference is made. The terminology includes the words abovespecifically mentioned, derivatives thereof and words of similar import.

Referring now to the drawings in detail, there is shown in FIGS. 1 and 2a coated inorganic substrate 12 having a multilayer antireflectioncoating in accordance with one preferred embodiment of the presentinvention, generally designated as 10. The antireflection coating 10 isapplied to a substrate 12 which is preferably inorganic. The substrateis preferably formed of a glass which can later be subjected to a heattreatment such as tempering. However, it will be understood, based onthis disclosure, that the coating may be applied to other inorganicsubstrates such as silica, ceramics and the like which require opticalcoatings and which are later subjected to heat treatment. While otherinorganic substrates may be used, the invention as described furtherbelow will be generally described with respect to the preferredembodiment including a glass substrate. Glasses which are preferablyused with and in the invention include ordinary window glass such as ⅛″or ¼″ ordinary window glass or float glass, or borosilicate glasses,glass ceramics and absorbing glasses. Absorbing glasses includecommercial glasses such as 70% transmission glass like Solorgray(available from PPG Industries) or Solarbronze (available from PPGIndustries), 50% transmission glass, 31% transmission privacy glass likeGraylite (available from PPG Industries) in color gray and 70%transmission privacy glass in color bronze.

One of the advantages of the present invention is that the coatings maybe applied to any size substrate, small or large, flat or shaped as wellas to the outside or inside of tubes or both. Thus the coatings may beapplied to large, un-cut pieces of glass for quick processing by dipcoating techniques, and later cut and/or trimmed for further heattreatment and/or tempering. Preferably, for forming tempered glass, thecoatings are applied to as large a piece of un-cut glass as possible tocontribute to the efficiency of the process.

The coatings are applied using dip coating or similar sol-gel techniquesand are preferably in a bilayer or multilayer design, preferably ofthree or four layers. However, it should be understood, based on thisdisclosure that additional layers may be provided for differentapplications. Two, three and four layer antireflection coatings formedby dip coating using other material combinations are known and aredescribed, for example, in H. A. Macleod, “Thin Film Optical Filters,”Adam Hilger, Ltd., Bristol 1985. The coatings are designed to providespecific indices of reflection for different applications to deliverrequired optical properties. Indices of reflection are materialconstants. The index of refraction of a material, the amounts of amaterial, the combinations of materials and layer thicknesses all affectthe optical properties of the resulting system. Bilayer coatingsgenerally have an M/L design which includes an inner layer of a materialhaving a middle level index of refraction (M layer) and a layer of amaterial of a low level index of refraction (L layer) as an outer layer.Such designs are useful, for example, with laser optic applications.Four layer systems generally have a H/L/H/L design and include an innerlayer formed of a material of a high level index of refraction (H layer)coated by a material of a low index of refraction (L layer) followed bya further H layer and L layer. Four layer coatings may be used fortechnical applications which need to accommodate a somewhat greateramount of light passing through the coating then for standardapplications. The coatings according to the present invention having M/Land H/L/H/L designs include materials as set forth below which,unexpectedly, allow for substantial retention of optical properties uponheat treatment.

In one embodiment, the multilayer coating is a “three layer low” designhaving an inner layer of a material of a middle level index ofrefraction, a middle layer of a high level index of refraction and anouter layer of a material of a low level index of refraction (M/H/L).Such three layer designs are typically used for window glass, pictureframes and the like as noted previously herein. As shown in FIG. 2, sucha multilayer coating design has three layers, an inner layer 14, amiddle layer 16 and an outer layer 18. The inner layer 14 has a middlelevel index of refraction in a cured coating of from about 1.54 to about1.90, preferably from about 1.68 to about 1.82, and is preferablyapplied in a thickness of λ/4 as measured in a direction transverselyacross the antireflection coating. If the coating is to have broad bandantireflective properties, λ is typically 550 nm. However, other valuesof λ are possible if different antireflective optical properties aredesired. The middle layer 16 is on the inner layer 14 as shown in FIG.2. The middle layer preferably has a material of a high level index ofrefraction of at least about 1.90 after curing and a preferred thicknessof 2×λ/4, i.e., λ/2. The outer layer 18 on the middle layer 16 as shownin FIG. 2 is preferably a material of low index of refraction of about1.54 or less after curing and is located on the “air side” of thecoating. The outer layer in the three layer low design preferably has athickness of λ/4. While the preferred thicknesses of the layers are λ/4,λ/2 and λ/4, respectively, it should be understood by one skilled in theart, that thickness may be varied for modifying or customizing opticalproperties for various coating applications.

The index of refraction is a known material constant. The index maydiffer for a given material depending upon crystalline structure or dueto modifications to the material. Using sol-gel coating techniques likedip coating, the index of refraction of a specific layer can also beadjusted by having different materials in a single layer as in thepresent invention. The index as measured in an already applied coatingis generally lower than the index of refraction as measured for the bulkmaterial due to imperfections in the network or crystal structure or thelevel of porosity of the coating. Coatings are typically porous to someextent and the degree of porosity depends on the material being used andthe coating technique.

In forming antireflective designs using the layers of the invention, theindex of refraction for the specific layers required is adjusted bymixing the materials described below and adjusting the layer thicknessesaccordingly. Such mixing techniques are not generally applicable invacuum coating techniques generally used for forming coatings onsubstrates. As such, the present invention offers advantages byproviding more flexibility in optical design. The specific selection ofmaterials in accordance with this invention assures the ability of thecoating to substantially retain optical properties after tempering, orotherwise heat treating the final coating.

Depending upon the reflection color desired, the coatings of theinvention may be provided in blue, green, purple or other reflectioncolors. While reflection of coatings in accordance with the inventiontypically exhibit very low levels of reflection, for example, less thanabout 1%, the reflection can be colored as noted above.

The inner layer 14 of the coating is applied such that it is preferablyin contact with the substrate 12. However, it will be understood thatthe substrate may already have one or more layers already appliedprovided such layers are capable of being heat processed. Examples ofstandard coatings which may already be on the glass include low Ecoatings, i.e., low emissivity coatings and/or conductive coatings suchas tin oxide and indium tin oxide (ITO) coatings which may or may not bedoped. With respect to the coating 10 in FIG. 2, the inner layer 14 ofthe invention which has a middle level index of refraction (M), includesat least two different metallic oxides which are capable of providing anindex of refraction of from about 1.54 to about 1.90, and preferablyfrom about 1.68 to about 1.82 after curing. While more than two oxidesmay be used, it is preferred that the at least two different oxidesinclude a metallic oxide having a high level index of refraction and amaterial having a low level index of refraction. Such a mixture ofmaterials provides the desired middle level index of refraction.Preferably one metallic oxide in the inner layer 14 is a low level indexof refraction material, for example, an oxide of silicon such as silicondioxide and/or an aluminum oxide. The high level index of refractionmaterial in the middle layer is preferably a metallic oxide or mixtureof oxides such as at least one oxide of titanium, zirconium, lanthanum,tantalum and/or niobium. Preferably, the high level material iszirconium oxide or one of the following combinations: zirconium oxideand niobium oxide; zirconium oxide and lanthanum oxide; zirconium oxideand tantalum oxide; or zirconium oxide and niobium oxide and tantalumoxide.

As such, the inner layer 14, is formed of a mixture of any of the lowindex materials and high index materials noted above, and mostpreferably is a mixture of aluminum oxide and zirconium oxide, Althoughit is also preferred to use a mixture of aluminum oxide and one of theother preferred zirconium oxide combinations noted above. Prior tocoating the substrate, the substrate is preferably cleaned by anysuitable method known in the art or to be developed. The method ofcleaning is not critical. The cleaned substrate, for example, a flatsheet of glass, is then dipped in a solution containing the abovemixture of oxides to form an inner layer of a middle index ofrefraction.

The solution is preferably a solvent based dip-coating solutionincluding at least one alkanol and water. Preferably, stabilizingagents, acids and catalytic active agents are also provided. The oxidenetworks in the finished coatings are achieved using sol-gel chemistryand through reactions involving the use of preferably commerciallyavailable precursors. However, specialty chemical or synthesizedprecursors may also be used. For example, a zirconium oxide crystallinenetwork structure may be formed by reactions of a precursor such aszirconium alkoxy nitrates or, more preferably, an alkoxy zirconiumalkoxide such as tetrabutoxy zirconium. Niobium oxide may be formed byreactions of niobium chlorides or, more preferably, alkoxy niobiumchlorides such as diethoxy niobium trichloride or triethoxy niobiumdichloride. However, it will be understood, based on this disclosure,that using basic sol-gel chemistry, precursors may be varied to providethe desired end product such as a network of pure zirconium oxide ormixtures of oxides. Preferably, when zirconium oxide formation isintended, chlorine use in the precursor is minimized to avoid unwantedprecipitation of zirconium dichloride or, with other networks, to avoidany other unwanted precipitated byproducts in the coating solution,during the coating or in the final layers. Further exemplary precursorsinclude, for formation of an aluminum oxide network, aluminum chlorideor, more preferably, aluminum nitrate, and for formation of a siliconoxide network, alkoxysilanes such as tetraethoxysilane. While aluminumchlorides may be used, it is preferred to use aluminum nitrate oraluminum alkoxides in view of the minimizing of chlorides as notedabove. Analogous precursor compounds may also used for formation ofother metallic oxide crystalline network structures. For example,lanthanum nitrate, tetrabutoxy titanium, aluminum acetyl acetonate andniobium acetate and other suitable alkoxides, nitrates, acetates, andchelates of the desired coating metals as noted above.

In selecting a low index of refraction material for mixture of oxides inthe inner layer 14 or in the outer layer 18 as described further below,aluminum precursors are especially preferred because they provide theadvantage of extending the life of the coating solution in comparisonwith typical prior art precursors such as silicon oxide precursors forlow index materials. Such property is significant in that it allows forextended usefulness and stability of the coating solutions. Sinceprecursors are expensive raw materials in forming the coating solutions,and the coatings typically require hazardous disposal when no longeruseful, prolonging the life of the coating solutions is very beneficial.Prolonged life for coating solutions will decrease overall cost therebyproviding a more economical and commercially viable process as well asto decrease the overall environmental impact of the disposal of unusedor expired solutions. The life of the coating solutions using thealuminum precursors can be extended for up to six to nine months asopposed to an average of about three months for the life of coatingsolutions formed using precursors such as tetraethoxysilane.

The precursors are to be provided in dilute form, for example, about 20g/l, within the solvent matrix. It should be understood that the amountin solution may be varied provided the criteria for dilution aresatisfied, i.e., there must be sufficient precursor for providing thedesired amount of metallic oxide in the final coating, but the precursormust be sufficiently diluted to keep the precursor molecules separateduntil the solution is applied to the surface to avoid premature reactionin the coating solution such that the coating and network formingreactions occur principally on the substrate surface after coating.Water provided initially is in small amounts for providing minor amountsof reactive OH groups. The stabilizing agents, for example, acids suchas acetic acid, glycols, polyglycols and similar compounds are added insmall amounts sufficient to carry out the function of complexing aroundthe precursor molecules to stabilize the precursor molecules insolution, but to avoid reaction with the precursor. If an acid is used,it may also function to catalyze the condensation reactions which occurduring the coating process. Such acids may be any standard condensationreaction catalyst acid, such as hydrochloric or nitric acid as well asacetic acid, and are also added in small amounts. The acid is present ina somewhat greater amount if acid is being used both as a stabilizer anda catalyst.

For forming a coating having a two metallic oxide mixture in the innerlayer, the solution should have a ratio of molar equivalents of fromabout 10:90 to about 90:10 of the first metal (for example silicon oraluminum) to the second metal (for example, zirconium) to provide thedesired amount of aluminum and zirconium in the finished metallic oxidenetwork in the inner layer 14.

Solvents which may be used include alkanols such as methanol, ethanol,butanol and the like, as well as any other similar solvents which aresubstantially non-reactive with the precursor but which exhibit rapidevaporation rates. During the dipping and subsequent coating reactions,the solvent evaporates, causing the water, precursor and acid or othercatalyst concentration in the solution to rise significantly. Further,atmospheric moisture is absorbed which further enhances theconcentration of water in the system and contributes to catalyzing thereaction. The surface area of the solution is expanded as the solutionis spread over the substrate surface which enhances evaporation ofsolvent, increasing this effect and the enhanced probability of reactionof precursor molecules as the proximity and separation of precursormolecules decreases and contact between molecules increases. The waterand precursor molecules undergo reaction leaving a network of oxygen andmetallic atoms. Drying removes more of the remaining water and solvent.

The substrate is withdrawn from the solution and a thin film remains onboth sides of the surface of the substrate. The film begins to thin dueto evaporation of solvent. As the solvent evaporates there is a bufferzone of solvent vapor above the surface of the coating film closer tothe dipping solution. As the substrate moves away from the dippingsolution, the vapor buffer decreases exposing the coating solution toatmospheric moisture and increasing the rate of reaction.

The acids, if present, further catalyze the reactions and, as theirconcentration increases due to the evaporation of solvent, the pH beginsto decrease. The chemical reactions are complex and their mechanisms arenot fully understood. However, it is believed that the overall reactionrate is catalyzed by the changing, i.e., increasing, concentrations ofcomponents, the evaporation of solvent and the increase in waterconcentration as described above. The reactions occur in the zoneextending longitudinally along the substrate surface as the solvent isat least partially evaporated from the solution.

Once coated and removed from the dipping solution, the substrate isbaked at temperatures of from about 120° C. to about 200° C., andpreferably from about 120° C. to about 180° C. to at least partially,and preferably to substantially remove the remaining solvent and waterfrom the solution and form the inner layer 14 as a solid film. It ispreferred that the temperature for drying be effective, but kept as lowas possible. However, it will be understood by one of ordinary skill inthe art, that the temperature for drying may vary in some casesdepending upon the materials used in the reactive mixture. At this step,the coating may be subjected to a curing or firing in a furnace attemperatures in excess of 200° C. However, such firing is not necessaryat this point. Absent such firing, the layer is not fully cured bydrying when the middle layer of the material of high index of refraction(H layer) is applied on the inner layer. Without wishing to be bound bytheory, it is believed that any remaining —OH groups left in the innerlayer 14 will aid in bonding the inner layer 14 with the middle layer 16being applied to the inner layer.

After the inner layer 14 has been applied, the middle layer 16 (H layer)is coated on the inner layer 14 preferably using a similar dip coatingprocedure described above. The middle layer 16 includes a material of ahigh index of refraction of at least about 1.90. Preferably, the highindex layer includes an oxide of zirconium, such as zirconium dioxide,however, other high index of refraction materials may be combined withthe oxide of zirconium to adjust the index of refraction. The coatingsolution is formed as noted above using the same type of materials inthe above noted amounts, and using the same or similar sol-gel chemistryand reactions, but using precursors which will provide oxides ofzirconium, such as a tetrabutoxy zirconium precursor as described above.

Other high index of refraction materials may also be provided to themiddle layer to adjust the index of refraction if necessary. Suchmaterials include oxides of lanthanum, tantalum, and niobium and areadded to the coating solution using suitable sol-gel precursors as notedabove, provided such precursors are selected to avoid potentialprecipitation of unwanted byproducts as described above. Such materialsmay raise the index somewhat depending on the characteristics of thematerials selected and the molar ratio used. Preferably, from about 50%to about 100%, and more preferably from about 90% to about 100% molarequivalents of zirconium oxide are provided to the middle layer 16regardless of the addition of other materials.

The substrate 12 is then withdrawn from the solution as described aboveand a thin film remains on the outer surface of the inner layer 14 onboth sides of the coated substrate. After evaporation and reaction, acoating is formed having a network of zirconium oxide or a combinationof zirconium oxide and other oxides, preferably at least one oflanthanum, tantalum, niobium, or silicon.

Once coated and removed from the dipping solution, the substrate isagain baked at temperatures of from about 120° C. to about 200° C., andpreferably from about 120° C. to about 180° C. to preferablysubstantially remove remaining solvent and water from the solution andform the middle layer 16 as a solid film. While the coating can besubjected to firing in excess of 200° C. for fully curing the coating,such firing is not necessary at this point. Without wishing to be boundby theory, it is believed that some reactive —OH sites preferably remainfor aiding in bonding the middle layer 16 to the overlying outer layer18. Absent additional firing as noted above, the layer 16 is not fullycured and the outer layer 18 of the material of low index of refraction(L layer) is applied on the middle layer 16. As noted above, it isbelieved that any remaining —OH groups left in the middle layer 16 willaid in bonding with the outer layer being applied to the middle layer.

After the middle layer 16 has been applied, the outer layer 18 (L layer)is coated on the middle layer 16 preferably using a similar dip coatingprocedure described above. The outer layer 18 includes a material of alow index of refraction of preferably about 1.54 or less. Preferably,the low index layer includes at least one oxide of a low index ofrefraction such as oxides of aluminum and of silicon. Oxides ofzirconium may also be combined in this layer provided the mixture ofoxides provides the desired low index of refraction. Preferably, theouter layer is a mixture of the oxides of silicon and aluminum. As notedabove, the aluminum oxides provide significant improvement in coatingsolution stability such that they are preferred low index materials inthe invention. Preferably the precursor is an aluminum nitrate, althoughother suitable aluminum precursors may be used. A suitable siliconprecursor may be any silicon sol gel precursor, preferablytetraethoxysilane. The zirconium precursor, if zirconium is furtherprovided in a mixture of low index oxides, may be the zirconiumprecursors as described above. The coating solution is also formed asnoted above with respect to the inner and middle layers 14, 16 using thesame type of reaction components in the above noted amounts but with thepreferred low index precursors for the outer layer 18, and using thesame or similar sol-gel chemistry and reactions.

The oxides may be used in various combinations provided to adjust theindex of refraction of the outer layer 18. Materials which may raise theindex somewhat can also be provided, such as those noted above, if theindex is sufficiently low and the intention is to slightly raise theindex. Such adjustments are known in the art and will depend on thecharacteristics of the materials selected and the molar ratio used.While aluminum oxide or silicon oxide may be used alternatively, if usedin combination, preferably, from about 5% to about 50% molar equivalentsof aluminum oxide and from about 50% to about 95% molar equivalents ofsilicon oxide are provided to the outer layer 18 regardless of theaddition of other materials, such as zirconium and the like.

The substrate 12 is then withdrawn from the solution as described aboveand a thin film remains on the outer surface of the middle layer 16 onboth sides of the coated substrate. After evaporation and reaction, acoating is formed having a network of oxygen atoms and silicon, aluminumand/or zirconium or any other oxides provided for adjusting the index ofrefraction of the outer layer 18 to be about 1.54 or less.

Once coated and removed from the dipping solution, the substrate isagain baked at temperatures of from about 120° C. to about 200° C.,preferably from about 120° C. to about 180° C. to preferablysubstantially remove remaining solvent and water from the solution andform the outer layer 18 as a solid film. At this point, the layer is notfully cured, but is further heat treated.

After all three layers have been applied and subjected to intermediatedrying, the entire coating is then finally heat treated by firing themultilayer coating at higher temperatures, preferably from about 300° C.to about 450° C. However, it should be understood that thesetemperatures may be adjusted somewhat for different coating materials ormaterial combinations used. The final firing completes formation of thenetwork forming reactions which occur in the layers such that theremaining solvent, water and active —OH groups are substantially removedin favor of —O— link bridges in the network structure. The firing mayalso contribute to completion of the crystallization process occurringwithin the coating layers. The final coating, after firing is dense andstable. The multilayer antireflection coating 10 is preferably of atotal thickness measured in a direction transverse to the substratesurface of from about 100 nm to about 400 nm, more preferably from about190 nm to about 300 nm, depending upon the optical system desired.Preferably, the coatings substantially retain their optical qualitiesafter any subsequent heat treatment after firing, i.e., the index ofrefraction, wherein the coating thickness preferably varies no greaterthan about ±5 nm, more preferably no greater than about ±3 nm andpreferably undergoes no change at all after the coating is formed andsubjected to any subsequent heat treatment of the substrate, such astempering of glass. Examples of the ability of the multilayer coatingsto substantially retain optical properties after heat treatment may befound in the optical curve data provided below in Example 4. The use ofthe zirconium-based precursors and the inclusion of the zirconium oxidenetwork in the antireflection coatings as described herein unexpectedly,and substantially contributes to the capability to retain opticalproperties as described herein. Further, the use of aluminum-basedprecursors and the inclusion of the aluminum oxides in the layers of thecoatings as a low index material provides enhanced stability of theprecursor solutions and further contributes to the economic andcommercial feasibility of formation of such coatings for use in largescale production of heat treated inorganic substrates such as temperedglass.

The coated inorganic substrate, which is preferably formed on a largesubstrate for economical reasons, but which may also be formed onsmaller substrates, can now be cut, trimmed or shaped as desired and thecoated substrate may be further heat treated and still substantiallyretain its optical properties. Other uses and applications for thecoatings of the invention include use in bending or forming glasswithout significantly changing the optical properties of the coating,for example, antireflective coated glass may be bent to the shape of acomputer monitor or television screen to form the front surfaces whichare later glued onto the screens. In addition, the coatings may be usedin any application where the coated glass will be exposed to hightemperature processing such as formation of optical coatings that allowonly light of specific wavelengths to pass through while blocking otherwavelengths including cut-off filters, color conversion filters, colorfilters such as those used in the film industry as special effect lightfilters or in laser applications. If the substrate or the coating havesome absorption, the substrate will heat up during use, especially ifthe light source used is a high-energy light source such as a laser.With conventional coatings, such heat and/or high-energy light exposurewill shift the optical system properties. However, coatings according tothe invention will undergo little, and preferably no shift in opticalproperties.

Preferably, if a glass substrate is used, such as the glass substratesnoted above, and is to be formed into safety glass in accordance withthe preferred embodiment of the invention, the glass substrate is coatedas noted above, and then subjected to a tempering process. Tempering maybe conducted in accordance with any known tempering method or by anytempering method to be later developed. The technique for tempering isnot critical to the invention. However, it should be noted that thetempering procedure to achieve safety glass standards must be conductedat temperatures sufficient to induce the requisite stress in the glass.The tempered safety glass has numerous practical applications includingsafety glass for automobiles and other vehicles, picture frames, displaycases and computer screens. Typically tempering is performed inaccordance with ASTM-C-1048-97b which provides the standardspecification for Heat-Treated Flat Glass-Kind HS, Kind FT Coated AndUncoated Glass, although similar procedures known in the art or to bedeveloped may also be used.

The invention also includes an inorganic coated substrate 12 coated withthe multilayer coating 10 having an M/H/L design as described in detailabove and as shown in FIGS. 1 and 2. The coated substrate may be any ofthe substrates noted above which are preferred for use with themultilayer antireflection coating. Preferably, the coating substantiallyretains its optical properties when the inorganic substrate is glass andthe coated glass is subjected to tempering as described above.

As noted above, bilayer coatings and multilayer coatings having four ormore layers are also within the scope of the invention. With respect tothe bilayer antireflection coating 20, as shown in FIG. 3, the coatinghas an inner layer 22 having a middle index of refraction and may beformed in the same manner and, preferably, using the same materials andprocedures as described above with respect to the inner layer 14 in theM/H/L multilayer antireflection coating 10. As such, the inner layer 22in the bilayer coating 20 is an M layer which is intended for contactingan inorganic substrate 12′. Preferably, the substrates are glass. Theinner layer 22 preferably comprises an oxide of zirconium and at leastone oxide of a metal different from zirconium such that the index ofrefraction of the inner layer 22 is from about 1.54 to about 1.90 andpreferably from about 1.68 to about 1.82 after curing in a manner asdescribed above with respect to the inner layer 14.

The outer layer 24 of the bilayer antireflection coating 20 preferablyincludes a material having a low index of refraction, an L layer. Theouter layer 24 is preferably formed on the inner layer using the samesol-gel chemistry and coating techniques described above and as used forthe inner layer 22. The outer layer 24 may include any of the samematerials noted above with respect to the outer layer 18 of themultilayer antireflection coating 10. The bilayer antireflection coating20 having an M/L design and inorganic substrate 12′ coated with suchbilayer antireflection coating 20 according to the invention arepreferably formed, and may be heat treated by any of the techniquesdescribed above with respect to the multilayer antireflection coating10.

Formation of a multilayer antireflection coating 26 as shown in FIG. 4having four layers is preferably formed in an H/L/H/L design, that is,an inner layer 28 is formed for contact with an inorganic substrate 12″.The substrate for use with such layers, preferably for technicalapplications which are intended to allow more light to pass through,such as those described in H. K. Pulker, Coatings On Glass (ElsevierPublishing) 1984 and may be the substrates as described above.Preferably, the substrate 12″ is glass. The inner layer 28 is a layerhaving a high index of refraction, an H layer, and may be formed of thesame materials and using the same techniques as the middle layer 16 inthe multilayer coating 10 described above. Preferably, the inner layer28 includes at least one oxide which is an oxide of zirconium and has anindex of refraction of at least about 1.90 after curing.

A first middle layer 30 is formed on the inner layer 28 and ispreferably a layer of a low index of refraction of 1.54 or less aftercuring. The first middle layer 30 may be formed of any of the materialsand using the techniques described above with respect to the outer layer18 in the multilayer antireflection coating 10.

A second middle layer 32 which is preferably the same as the inner layer28 is formed on the first middle layer 30 such that the second middlelayer 32 is also a layer of a high index of refraction. It should beunderstood, based on this disclosure, that the oxides provided to theinner layer 28 and the second middle layer 32 may vary and may beadjusted with different materials, provided the index of refraction isat an acceptably high level.

An outer layer 34 is formed on the second middle layer 32. The outerlayer 34 is a layer of a low index of refraction, an L layer, and ispreferably the same as the first middle layer 30 noted above. However,as noted above with respect to layers 28 and 32, the outer layer 34 andthe first middle layer 30 may have different materials provided theyboth achieve an acceptably low index of refraction of preferably about1.54 or less. The present invention includes an antireflection coatedinorganic substrate which includes an inorganic substrate 12″ coatedwith the multilayer antireflection coating 26 as described herein.

The method of the present invention includes a method for making a heattreated antireflective coated inorganic substrate. The method includescoating an inorganic substrate, such as the substrates described above,with an inner layer of a middle index of refraction such as thosedescribed above with respect to the inner layer 14 in the multilayerantireflection coating 10. A middle layer, such as the middle layer 16of a high index of refraction is coated on the inner layer 14. An outerlayer 18 of a low index of refraction is coated on the middle layer 16.The layers as coated on the inorganic substrate 12 are then heattreated. This same method, within the scope of the invention may includecoating for formation of a bilayer and multilayer coating of four ormore layers. The layers in the method of the invention are preferablyall coated on the substrate and each other using the sol-gel chemistryand dipping techniques described in detail above. The heat treatment maybe any of those mentioned with respect to the antireflection coating 10.

The invention will now be described based on the following non-limitingexamples:

EXAMPLE 1

An M-layer solution was formed by combining 500 ml of 95% ethanol and 80ml of tetrabutoxy zirconium by mixing in a beaker while stirring. To themixture, 50 ml of acetic acid were added with continuous stirring forfour hours at room temperature. After four hours, 90 g of aluminumnitrate (Al(NO₃)₃.9H₂O) were dissolved in 100 ml of ethanol and wereadded to the solution of tetrabutoxy zirconium, ethanol and acetic acid.The solution was stirred for another 24 hours at room temperature andthan ethanol was added to the solution until a total volume of 1 literwas reached. The solution was used to form a coating having a refractiveindex of 1.76.

EXAMPLE 2

An H-layer solution was formed by mixing 500 ml of 95% ethanol and 180ml of tetrabutoxy zirconium in a beaker while stirring at roomtemperature. To this mixture, 50 ml acetic acid were added. The mixturewas stirred for four hours at room temperature and then ethanol wasadded to the solution until a volume of 1 liter was reached. Thesolution formed a coating having an index of refraction of 1.95.

EXAMPLE 3

An L-layer solution was formed by mixing 160 ml of ethanol, 93 ml oftetraethoxysilane, 54 ml of DI water and 1 ml of HCl (35%) at roomtemperature while stirring. During stirring at room temperature, theviscosity was measured every hour. When the viscosity reached a value of3.0-3.2 centistokes, a second solution of 2 g of aluminum nitrate(Al(NO₃)₃.9H₂O dissolved in 50 ml of ethanol was added to the solutionof tetraethoxysilane, DI water and HCl. Both solutions were then mixedwell and ethanol was added to the solution until a total volume of 1liter was reached. The solution formed a coating having an index ofrefraction of 1.46.

EXAMPLE 4

A piece of float-glass having a thickness of 3 mm and a size of 44 in.(111.76 cm)×66 in. (162.56 cm) was cleaned. The glass was dipped in anM-solution made in accordance with Example 1 and was withdrawnvertically from that solution at a rate of 6 mm/s. After removing theglass it was dried in an oven at 150° C. for 5 minutes. After allowingthe glass to cool to room temperature, it was dipped into an H-layersolution formed in accordance with Example 2 and withdrawn verticallyfrom that solution at a rate of 8 mm/s. The glass was then dried againin an oven at 150° C. for 5 minutes. The glass was allowed to cool toroom temperature and was then dipped into an L-layer solution formed inaccordance with Example 3 and withdrawn vertically at a rate of 8 mm/s.The percent reflectance was measured at 10 nm intervals from 400 nm to690 nm and the data are as shown below in Table 1 and as representedgraphically in FIG. 5. In FIG. 5, curve A represents the desired targetvalue to be achieved and is measured before tempering the coated glass.Curve B in FIG. 5 represents the percentage reflectance after tempering.As can be seen in FIG. 5 there is little shift in the curve before andafter tempering and optical quality is retained.

TABLE 1 % Reflectance % Reflectance Wavelength Before After WavelengthBefore After (nm) (A) (B) (nm) (A) (B) 410 1.87 1.64 550 1.29 1.60 4201.05 0.94 560 1.15 1.47 430 0.65 0.62 570 0.98 1.30 440 0.56 0.58 5800.81 1.11 450 0.67 0.72 590 0.66 0.93 460 0.89 0.96 600 0.53 0.78 4701.09 1.19 610 0.44 0.67 480 1.29 1.42 620 0.39 0.60 490 1.45 1.60 6300.37 0.54 500 1.55 1.73 640 0.38 0.52 510 1.57 1.79 650 0.43 0.54 5201.54 1.79 660 0.52 0.60 530 1.49 1.76 670 0.66 0.72 540 1.41 1.70 6800.85 0.89 690 1.09 1.11

As can be seen from the above data, while various three layer low,bilayer and four layer design coatings, as well as higher numbers oflayers of coatings are available in the art, the particular systems andtechniques of the present invention provide unique stability andsubstantially retain optical properties after heat treatments such astempering. The claimed invention further is economical and easy to usefor large scale production of antireflective coated substrates usinglarge sized substrates due to the ability to dip coat the substrateprior to cutting and tempering. The invention further does not requireexpensive equipment and techniques such as sputtering. Therefore, theantireflection coatings, antireflection coated inorganic substrates andthe method of the invention satisfy a need in the art for economicalformation of antireflection coated substrates which may be large insize, without the need for expensive procedures and allowing cutting andtrimming after coating while substantially maintaining opticalproperties. The coatings of the invention as formed by the method of theinvention are of optical quality and show substantial uniformity anddurability. They are very heat stable and substantially retain theiroptical properties and achieve optical quality even after heattreatments such as tempering processes for forming safety glass.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above, including providing additionalcoating layers above or below the multilayer coating of the invention,without departing from the broad inventive concept thereof It isunderstood, therefore, that this invention is not limited to theparticular embodiments disclosed, but it is intended to covermodifications within the spirit and scope of the present invention asdefined by the appended claims.

We claim:
 1. A method for making a heat treated antireflective coatedinorganic substrate comprising: (a) coating an inorganic substrate withan inner layer comprising a mixture of at least one first oxide of ametal selected from the group consisting of titanium, zirconium,lanthanum, tantalum, and niobium and at least one second oxide of ametal selected from the group consisting of silicon and aluminum to forman inner layer; (b) coating the inner layer with a middle layercomprising an oxide of zirconium; (c) coating the middle layer with anouter layer of at least one oxide of a metal selected from the groupconsisting of silicon and aluminum; and (d) heat treating the coatedinorganic substrate wherein said heat treatment comprises tempering. 2.The method according to claim 1, wherein the inorganic substrate is aglass substrate.
 3. The method according to claim 1, wherein each of theinner, middle and outer layers are successfully applied by dip coatingprocess.
 4. The method according to claim 1, wherein step (a) furthercomprises coating the inorganic substrate with a solvent solutioncomprising the mixture of the at least one first oxide and the at leastone second oxide, at least partially evaporating solvent in the solventsolution comprising the mixture, and baking the coated inorganicsubstrate at a temperature sufficient to substantially remove liquidfrom the solution and to form the inner layer.
 5. The method accordingto claim 4, wherein step (b) further comprises coating the inner layerwith a solvent solution comprising the oxide of zirconium, at leastpartially evaporating solvent in the solvent solution comprising theoxide of zirconium, and baking the coated inorganic substrate at atemperature sufficient to substantially remove liquid from the solutionand to form the middle layer.
 6. The method according to claim 5,wherein step (c) further comprises coating the middle layer with asolvent solution comprising the at least one metallic oxide, at leastpartially evaporating solvent in the solvent solution comprising the atleast one oxide, and baking the coated inorganic substrate at atemperature sufficient to substantially remove liquid from the solutionto form the outer layer.
 7. The method according to claim 6, whereinstep (c) further comprises heating the coated inorganic substrate at anincreased temperature after baking the outer layer to completelayer-forming reactions occurring within the inner, middle and outerlayers of the coating.
 8. The method according to claim 1, furthercomprising cutting or trimming the inorganic substrate to apredetermined shape after step (c) and before step (d).
 9. A method formaking a heat treated antireflective coated inorganic substratecomprising: (a) coating an inorganic substrate with an inner layercomprising a mixture of at least one first oxide of a metal selectedfrom the group consisting of titanium, zirconium, lanthanum, tantalum,and niobium and at least one second oxide of a metal selected from thegroup consisting of silicon and aluminum to form an inner layer; (b)coating the inner layer with a middle layer comprising an oxide ofzirconium; (c) coating the middle layer with an outer layer of at leastone oxide of a metal selected from the group consisting of silicon andaluminum; and (d) heat treating the coated inorganic substrate, whereinthe coated inorganic substrate substantially retains optical propertiesafter the heat treatment.
 10. The method according to claim 9, whereinthe inorganic substrate is a glass substrate.
 11. The method accordingto claim 9, wherein step (d) comprises heat treating the inorganicsubstrate by tempering.
 12. The method according to claim 9, whereineach of the inner, middle and outer layers are successively applied by adip coating process.
 13. The method according to claim 9, wherein step(a) further comprises coating the inorganic substrate with a solventsolution comprising the mixture of the at least one first oxide and theat least one second oxide, at least partially evaporating solvent in thesolvent solution comprising the mixture, and baking the coated inorganicsubstrate at a temperature sufficient to substantially remove liquidfrom the solution and to form the inner layer.
 14. The method accordingto claim 13, wherein step (b) further comprises coating the inner layerwith a solvent solution comprising the oxide of zirconium, at leastpartially evaporating solvent in the solvent solution comprising theoxide of zirconium, and baking the coated inorganic substrate at atemperature sufficient to substantially remove liquid from the solutionand to form the middle layer.
 15. The method according to claim 14,wherein step (c) further comprises coating the middle layer with asolvent solution comprising the at least one metallic oxide, at leastpartially evaporating solvent at the solvent solution comprising the atleast one oxide, and baking the coated inorganic substrate at atemperature sufficient to substantially remove liquid from the solutionto form the outer layer.
 16. The method according to claim 15, whereinstep (c) further comprises heating the coated inorganic substrate at anincreased temperature after baking the outer layer to completelayer-forming reactions occurring within the inner, middle and outerlayers of the coating.
 17. The method according to claim 9, furthercomprising cutting or trimming the inorganic substrate to apredetermined shape after step (c) and before step (d).